This year has been one of the worst for tornadoes in decades as nearly 1,200 twisters have touched down in several states in the U.S.

Four of these tornadoes have been estimated to have been an EF-5, according to the National Oceanic and Atmospheric Administration.

More than 500 people have been killed, which is the highest death toll reported as a result of severe weather since 1953.

Live Science.com reported that this deadly outbreak of severe storms is a result of cold air that was left over from this year's snowstorms. Because of some rare changes in atmospheric winds, the cold air has mixed with unusually hot conditions in the Southwest and that's acted as fuel for these massive storms.

However, scientists say it's important to take a hard look at the numbers before jumping to any conclusions about the sheer numbers of tornadoes in the United States, and whether those numbers are going up.

"Just because we've seen an increase in the number of tornadoes doesn't mean there has actually been an increase in the number of tornadoes," Greg Carbin, the warning coordination meteorologist with the Storm Prediction Center in Norman, Okla, told the science website.

Decades ago, when the country was more sparsely populated - and not everyone had a camera-equipped cell phone - there were simply fewer people around to spot and report tornadoes, Carbin said.

In addition, Carbin said, many initial tornado tallies include tornadoes that are counted more than once.